1. Field of the Invention
This invention relates generally to the art of servomechanisms and more particularly to a new and improved circuit to minimize the acquisition transients of such devices.
2. Description of the Prior Art
Applications exist in phase locked loops and in other servosystems for amplifiers which provide a control voltage, in the absence of an input signal, that is at a level near the control amplifier lock-up voltage to maintain the acquisition time within a desired interval. This type of operation is generally desired when a servo is faced with a situation wherein lock is broken frequently and a fast reacquisition time is a requirement. A communication system employing phase shift keying (PSK) modulation, wherein information is carried as phase discontinuities of a carrier signal, is an example of such a system. PSK communication systems employ phase locked loops to extract the carrier from the discontinuously phase modulated signal, and it is necessary to limit the reacquisition transients of the phase lock loop in order to minimize the probability of information error. Prior art methods of performing the necessary limiting have employed Zener diodes and adjustable voltage clamps which discharge the capacitors in the loop filter in the process of maintaining a minimum control voltage after lock has been lost. For the system to reacquire smoothly and track, starting at the minimum voltage maintained, the filter capacitors must maintain the charges that would have been present at that voltage had track not been lost. When this charge is not maintained, erratic initial tracking and acquisition results until the proper charge has been re-established. This invention provides a means by which the desired filter charge is maintained at the established voltage bound until the input signal again comes within acquisition range.